(Adapted from the Applicant's Abstract): Induced abortion has been associated with an increased risk of breast cancer, but only a small number of well-conducted studies have reported on the relationship. A key challenge to the findings reported in those studies concerns the possible inaccuracy of self-reported information on induced abortion: Breast cancer cases may have been more willing than women without a life-threatening illness to report their history of abortion, resulting in spurious, positive findings. We propose a population-based case- control study to examine the relationship. To increase the accuracy of information on induced abortion, the study will be conducted in Shanghai, China, where induced abortion is not stigmatized and where family planning records document reproductive history. All women aged 54 years and younger, newly diagnosed with breast cancer during 1997 through mid-1999 and resident in Shanghai, will be identified through the population-based Shanghai Cancer Registry. The same number of age- matched controls will be randomly selected from the population of the geographic area. Approximately 1,470 cases and 1,470 controls will be interviewed to ascertain the number, timing, and outcome of all pregnancies; other reproductive, menstrual, and contraceptive characteristics; family history of breast cancer; and other risk factors for breast cancer. In addition, we will abstract the reproductive records dept for each participant by the Family Planning Committee. They contain a detailed history of each pregnancy and will be used to enhance the accuracy of the information on induced abortion. Medical records will be requested to obtain information on a history of benign breast disease and use of breast-cancer screening services. The association of induced and spontaneous abortion with breast cancer risk will be examined in relation to age at abortion, parity, and timing of the abortion (before and after first full-term pregnancy, interval to next full-term pregnancy, gestational age.) Should a relationship with induced abortion be found, it will be possible to provide women who are contemplating abortion better information on the possible risks of the procedure in relation to age, parity, and timing of the procedure.